An optical add/drop device such as a reconfigurable optical add-drop multiplexer (ROADM) is equipped with a wavelength selective switch (WSS) that adds/drops an optical signal of any desired wavelength to/from a multiple-wavelength optical signal as disclosed in Japanese Patent Application Publication Nos. 2011-160162 and 2017-11506. As the filter passbands corresponding to the respective selectable wavelengths are set in the wavelength selective switch, the passband of an optical signal becomes narrower (PBN: PassBand Narrowing) due to accumulation of filter passbands every time passing through the optical add/drop device.
In a backbone network that performs long-distance transmission, for example, the number of signals to be subjected to adding/dropping is small, and accordingly, the influence of passband narrowing is small. In a metropolitan area network that performs short-distance transmission in a metropolitan area or the like, on the other hand, the number of signals to be subjected to adding/dropping is large, and therefore, the influence of passband narrowing is large.